(1) Field of the Invention
The present invention relates to an output port switching device in N-WDM (N-channel Wavelength Division Multiplexing) system, and for example, to an output port switching device in N-WDM system which is provided with a switching backup output port having a laser diode serving as a backup light source, and performs a control operation so that the wavelength of an optical signal emitted from the backup output port is coincident with the wavelength of an optical signal to be emitted from a trouble-occurring port.
(2) Description of the Related Art
A conventional output port switching device in N-WDM system is generally constructed as shown in FIGS. 3 to 6. That is, FIGS. 3 to 6 show the basic construction of n-waves N-WDM devices of a transmission side, a reproducing relay and a reception side respectively, and FIG. 6 is a block diagram showing the internal construction of a transmission device, a reception device and a reproducing relay which constitute an N-WDM system (prior art 1).
In FIGS. 3 to 6, the N-WDM transmission device 4 receives optical signals of n from terminals 7 at the respective ports of n, and temporarily converts each optical signal to an electrical signal by using an optical/electrical converter (O/E) 11. The electrical signals thus obtained are converted to optical signals of n having wavelengths .lambda.1 to .lambda.n again by using an electrical/optical converter (E/O) 12. The respective wavelengths are subjected to n-wave multiplexing by using a multiplexer 13. The n-wave multiplexed signal thus obtained is amplified to a predetermined output level by POST-AMP 16 as shown in FIG. 4, and then transmitted to an optical fiber transmission line 8.
The N-WDM reception device 3 receives the n-wave multiplexed signal which is weakened due to transmission loss as shown in FIG. 5, amplifies the input n-wave multiplexed signal by PRE-AMP 24, and then demultiplexes the amplified signal into optical signals of n having wavelengths .lambda.1 to .lambda.n by using a demultiplexer 25. Each split signal is converted to an electrical signal by O/E 26, then converted to an optical signal again by E/O 27, and then transmitted from each port to each terminal 10 as shown in FIG. 6.
The reproducing relay 2 used in a long-distance transmission system has such a structure that the portion extending from the input of the transmission line to O/E 26 in the reception device 3 is connected to the portion extending from E/O 12 to the output of the transmission line in the transmission device.
In the high-speed transmission line, it has been generally adopted to change a transmission line being used to a normal transmission line when some trouble occurs in the transmission line because if a trouble occurs in the transmission line, it brings a critical effect. In an optical transmission line, deterioration of an optical device such as a laser diode or the like at the transmission side is one factor which causes some trouble in the transmission line.
The N-WDM system in which optical signals having plural different wavelengths in the same wavelength band are subjected to wavelength division multiplexing needs laser diodes whose number is equal to at least the number of the wavelengths to be multiplexed, and this makes the trouble-occurring probability higher than that in a normal system which performs no wavelength division multiplexing. In addition, an optical device is higher in price than a normal electrical device, and thus provision of a backup transmission line for switching makes the cost higher than the normal system.
Further, in the general digital transmission line (not limited to the N-WDM system), when a switching operation to another backup transmission line is performed, the switching timing would be generally shifted, or even if the timing is matched, the frame synchronization at the reception side would be deviated due to a delay difference between the transmission lines, which causes such a trouble that a signal cutoff state occurs instantaneously.
Japanese Laid-open patent application no. sho-59-86929 (Prior Art 2) has proposed "Optical Transmission System" in order to solve the above problems. As disclosed in the prior art 2, a backup E/O having a wavelength AN different from the wavelengths of the other optical signals and a multiplexer are provided to the transmission side, and a backup O/E having a wavelength .lambda.N and a demultiplexer are provided to the reception side. In this publication, the switching operation of the transmission line is not performed, and the trouble-occurring wavelength is switched to the backup wavelength .lambda.N.
FIG. 7 is a system block diagram showing the construction of the system of the prior art 2. In FIG. 7, the input of each port of the transmission device 5 is branched, and one input is directly input to E/O 12 while the other input is input to a change-over switch 28. For example when a trouble occurs in an i-th port of the transmission device 5 and transmission of a signal of wavelength .lambda.i is stopped, the change-over switch 28 selects the signal from the i-th port and transmits it to the backup E/O 12. The signal from the i-th port is converted to an optical signal of wavelength .lambda.N. The optical signal thus converted is subjected to the wavelength division multiplexing together with the other normal signals by the multiplexer 13, and then transmitted to the transmission line.
In the reception device 6, the optical signal of wavelength .lambda.N is optoelectrically converted by O/E 26, and then input to a change-over switch 29. The change-over switch 29 connects the input signal to a non-instantaneous-cutoff switch 30. The non-instantaneous-cutoff switch 30 switches the output of the reception device 6 from a trouble-occurring signal to a signal transmitted through the backup light source.
However, the first problem of the prior art 1 resides in that the cost is high. Further, the prior art 2 has a lower cost than the transmission line switching system, however, a backup optical device must be provided to each of the transmission and reception devices. This is because the wavelength of the backup E/O of the transmission device is different from the wavelength of any optical signal under the normal state. Accordingly, in the system construction which needs a reproducing relay, backup E/O and O/E must be provided in the reproducing relay, and this makes the cost higher.
Further, there is a second problem in the use efficiency of the wavelength band and the wavelength interval between the optical signals which are subjected to the wavelength division multiplexing. That is, the number of times at which the wavelength division multiplexing operation of N-WDM is performed is continued to increase, and the wavelength interval must be shortened in order to increase the number of times of the multiplexing in a limited wavelength band. However, as the wavelength interval is reduced, the wavelength splitting (division) technique becomes more difficult. Therefore, it is required not to increase needless wavelengths even if it is only one wavelength.